Hydrogenated ZnO thin films with improved electro-optical properties
Diana Gaspar
*
, Luís Pereira, Elvira Fortunato, and Rodrigo Martins
CENIMAT/I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e
Tecnologia, FCT, Universidade Nova de Lisboa and CEMOP-UNINOVA, 2829-516
Caparica, Portugal.
Zinc oxide (ZnO) is a wide band gap (Eg ≈ 3.3 eV) semiconductor that has been focus of
research for many decades due to its multi-functionality that turns it possible to be used in a
plethora of applications, being considered an attractive material as transparent conductive
oxide (TCO), since it owns excellent optoelectronic properties, high transmittance and good
electric conductivity.
Hydrogen is known as amphoteric impurity in semiconductors; however the role of this
element in the ZnO is not consensual and clearly understood. Theoretical studies defend the
non-amphoteric role of the H
2
due the enhancement of conductivity of ZnO.
In this work we report on the production of hydrogenated ZnO
thin films deposited by rf
reactive magnetron sputtering varying hydrogen dilution, from 0 to 2%, in the gas mixture.
This allowed a comprehensive analysis of the structure, morphology and composition of the
films and how it correlates with the electro-optical properties, in order to better understand
the effective role of hydrogen in undoped ZnO.
The transparency and conductivity of the in situ hydrogenated zinc oxide films (ZnO:H)
improved with the addition of H
2
to the sputtering atmosphere, where the resistivity dropped
one order of magnitude (from 3x10
-2
to 2.8x10
-3
Ω cm) for an optimal a H
2
dilution of 1.5%.
The effect of hydrogen in the conductivity of undoped ZnO thin films has been commonly
attributed to the shallow donor behavior. However, we observed that the improvement in the
conductivity came essentially from the improvement of mobility that reached 47.1 cm
2
V
-1
s
-1
for a carrier concentration of 4.4x10
19
cm
-3
. This improvement on the mobility happen with
an effective H
2
incorporation along the entire thickness of the sputtered films, and attributed
to the fill/passivation of grain boundaries traps states and other defects, such as zinc
vacancies and oxygen vacancies.
O 59
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